The frequency of meteorite falls: Comments on two conflicting solutions to the problem

Abstract— Recent estimates of the current rate of meteorite falls have been derived from camera network observations and from the statistics of the recovery of small fragments in Roosevelt County, New Mexico. The results are discordant. The integrated sky coverage for the camera data is an order of magnitude greater than the long exposure over small areas in New Mexico, yet the number of inferred events is comparable. We examine potential problems and find no effects other than random ones to bias the camera data. New data on the total number of suitable fireballs indicate that the flux from the camera network is already close to the upper limit imposed by the count of over 700 fireballs. By contrast, the calibration of the decay time for small fragments in New Mexico appears insecure and could account for a factor-of-three discrepancy. The large area of a typical strewn field relative to the small search areas is also a serious problem that remains uncalibrated. There are problems with the application of either the camera results or the New Mexico data to the statistics of recoveries in Antarctica since the Antarctic search areas are not free from the strewn-field problem. Since there is no evidence for a substantial change in the infall rate in intervals less than the ages of the collecting surfaces, we believe the camera network flux data remain the best estimate for the arrival rate of small meteorites on Earth.     

The impact of the October–November 2000 floods on contaminant metal dispersal in the River Swale catchment,North Yorkshire,UK

During the autumn of 2000, large areas of England and Wales were affected by severe flooding, which caused widespread disruption and significant damage to property. This study attempts to determine the impact of these flood events on contaminated sediment dispersal and deposition in the River Swale catchment, Yorkshire, UK, where lead and zinc were extracted and processed in large quantities during the nineteenth century. Seventy samples of overbank and channel‐edge sediments were collected at 35 sites along the River Swale. Inductively coupled plasma‐mass spectrometry was used to measure contaminant metal concentrations in the 2000–63 µm (sand) and <63 µm (silt and clay) size fractions. In both the channel‐edge and overbank sediments collected from the upper and middle reaches of the River Swale, concentrations of lead, zinc and cadmium were found to exceed MAFF guidelines. Highest concentrations correspond to the input of contaminated material from intensively mined tributaries, and elevated levels can be observed 5–10 km downstream of these inputs. This indicates that the remobilization of contaminated material during major flood events is potentially a serious problem for activities such as agriculture that utilize the Swale floodplain. Copyright © 2003 John Wiley & Sons, Ltd.     

Rotational mixing in early-type main-sequence stars

We present N -body simulations of galaxy groups embedded in a common halo of matter. We study the influence of the different initial conditions upon the evolution of the group and show that denser configurations evolve faster, as expected. We then concentrate on the influence of the initial radial density profile of the common halo and of the galaxy distribution. We select two kinds of density distributions, a singular profile (modelled by a Hernquist distribution) and a profile with a flat core (modelled by a Plummer sphere). In all cases we witness the formation of a central massive object owing to mergings of individual galaxies and to accretion of stripped material, but both its formation history and its properties depend heavily on the initial distribution. In Hernquist models the formation is caused by a 'burst' of mergings in the inner parts, owing to the large initial concentration of galaxies in the centre. The merging rate is much slower in the initial phases of the evolution of a Plummer distribution, where the contribution of accretion to the formation of the central object is much more important. The central objects formed within Plummer distributions have projected density profiles which are not in agreement with the radial profiles of observed brightest cluster members, unless the percentage of mass in the common halo is small. In contrast, the central object formed in initially cusped models has projected radial profiles in very good agreement with those of brightest cluster members, sometimes also showing luminosity excess over the r ^1/4 law in the outer parts, as is observed in cD galaxies.     

The Kwakshua Watersheds Observatory,central coast of British Columbia,Canada

The Kwakshua Watersheds Observatory (KWO) is an integrative watersheds observatory on the coastal margin of a rain-dominated bog-forest landscape in British Columbia (BC), Canada. Established in 2013, the goal of the KWO is to understand and model the flux of terrestrial materials from land to sea – the origins, pathways, processes and ecosystem consequences – in the context of long-term environmental change. The KWO consists of seven gauged watersheds and a network of observation sites spanning from land to sea and along drainage gradients within catchments. Time-series datasets include year-round measurements of weather, soil hydrology, streamflow, aquatic biogeochemistry, microbial ecology and nearshore oceanographic conditions. Sensor measurements are recorded every 5 min and water samples are collected approximately monthly. Additional observations are made during high-flow conditions. We used remote sensing to map watershed terrain, drainage networks, soils and terrestrial ecosystems. The watersheds range in size from 3.2 to 12.8 km², with varying catchment characteristics that influence hydrological and biogeochemical responses. Despite local variation, the overall study area is a global hotspot for yields of dissolved organic carbon, dissolved organic nitrogen and dissolved iron at the coastal margin. This observatory helps fill an important gap in the global network of observatories, in terms of spatial location (central coast of BC), climate (temperate oceanic), hydrology (very high runoff, pluvial regime), geology (igneous intrusive, glacially scoured), vegetation (bog rainforest) and soils (large stores of organic carbon).     

Assessing characteristics of Mediterranean explosive cyclones for different data resolution

A comparison of two objective climatologies of explosive cyclones in the Mediterranean region is performed. The results are derived from two different mean sea-level pressure reanalysis data resolutions, but from the same assimilation model, in order to quantify the pure impact of higher resolution on the identification and characteristics of explosive cyclones, when the assimilation model is the same. The explosive cyclones were identified with the aid of the Melbourne University automatic cyclone finding and tracking scheme over a 40-year period, using the 6-hourly analyses of ERA-40 mean sea-level pressure (MSLP) on: (a) 2.5?×?2.5 and (b) 1?×?1 latitude–longitude grid. The comparison of the two datasets revealed the significant role of the increase in spatial resolution of MSLP data on the identification and tracking process, and the number of the explosive cyclones in the high-resolution dataset is almost four times greater than the respective one in the lower resolution dataset. However, the comparison of explosive cyclone characteristics, including spatial and temporal variations of explosive deepening, revealed differences in the geographical distribution of the location of the maximum explosive deepening and average explosive cyclone Laplacian of the central pressure. These differences are due to the identification in the higher resolution set of smaller scale and secondary explosives along the strongly baroclinic northern Mediterranean boundaries, south of the Alps and the Pyrenees. Explosive deepening appears a bias to the daytime period from 12 to 18 Coordinated Universal Time (UTC) for both datasets, which is more prominent in the LR dataset. Statistically significant difference of pressure tendency between the two datasets appear for the daytime period from 06 to 12 UTC, accounting for better representation of orographic forcing in the HR dataset.     

Pacific Atmospheric Sulfur Experiment (PASE): dynamics and chemistry of the south Pacific tropical trade wind regime

The Pacific Atmospheric Sulfur Experiment (PASE) was a comprehensive airborne study of the chemistry and dynamics of the tropical trade wind regime (TWR) east of the island of Kiritibati (Christmas Island, 157º, 20?? W, 2º 52?? N). Christmas Island is located due south of Hawaii. Geographically it is in the northern hemisphere yet it is 6?C12º south of the intertropical convergence zone (ITCZ) which places it in the southern hemisphere meteorologically. Christmas Island trade winds in August and September are from east south east at 3?C15 ms^?1. Clouds, if present, are fair weather cumulus located in the middle layer of the TWR which is frequently labeled the buffer layer (BuL). PASE provided clear support for the idea that small particles (80 nm) were subsiding into the tropical trade wind regime (TWR) where sulfur chemistry transformed them to larger particles. Sulfur chemistry promoted the growth of some of these particles until they were large enough to activate to cloud drops. This process, promoted by sulfur chemistry, can produce a cooling effect due to the increase in cloud droplet density and changes in cloud droplet size. These increases in particle size observed in PASE promote additional cooling due to direct scattering from the aerosol. These potential impacts on the radiation balance in the TWR are enhanced by the high solar irradiance and ocean albedo of the TWR. Finally because of the large area involved there is a large factional impact on earth??s radiation budget. The TWR region near Christmas Island appears to be similar to the TWR that persists in August and September, from southwest of the Galapagos to at least Christmas Island. Transport in the TWR between the Galapagos and Christmas involves very little precipitation which could have removed the aerosol thus explaining at least in part the high concentrations of CCN (??300 at 0.5% supersaturation) observed in PASE. As expected the chemistry of sulfur in the trade winds was found to be initiated by the emission of DMS into the convective boundary layer (BL, the lowest of three layers). However, the efficiency with which this DMS is converted to SO₂ has been brought into further question by this study. This unusual result has come about as result of our using two totally different approaches for addressing this long standing question. In the first approach, based on accepted kinetic rate constants and detailed steps for the oxidation of DMS reflecting detailed laboratory studies, a DMS to SO₂ conversion efficiency of 60?C73% was determined. This range of values lies well within the uncertainties of previous studies. However, using a completely different approach, involving a budget analysis, a conversion value of 100% was estimated. The latter value, to be consistent with all other sulfur studies, requires the existence of a completely independent sulfur source which would emit into the atmosphere at a source strength approximately half that measured for DMS under tropical Pacific conditions. At this time, however, there is no credible scientific observation that identifies what this source might be. Thus, the current study has opened for future scientific investigation the major question: is there yet another major tropical marine source of sulfur? Of equal importance, then, is the related question, is our global sulfur budget significantly in error due to the existence of an unknown marine source of sulfur? Pivotal to both questions may be gaining greater insight about the intermediate DMS oxidation species, DMSO, for which rather unusual measurements have been reported in previous marine sulfur studies. The 3 pptv bromine deficit observed in PASE must be lost over the lifetime of the aerosol which is a few days. This observation suggests that the primary BrO production rate is very small. However, considering the uncertainties in these observations and the possible importance of secondary production of bromine radicals through aerosol surface reactions, to completely rule out the importance of bromine chemistry under tropical conditions at this time cannot be justified. This point has been brought into focus from prior work that even at levels of 1 pptv, the effect of BrO oxidation on DMS can still be quite significant. Thus, as in the case of DMS conversion to SO₂, future studies will be needed. In the latter case there will need to be a specific focus on halogen chemistry. Such studies clearly must involve specific measurements of radical species such as BrO.